Finding an effective way to periodically inspect storage tanks for service induced damage is a continuing problem. In the case of waste storage applications, recent legislative actions have required that these vessels operate reliably over their intended lifetimes. The ability to demonstrate tank integrity for very long times demands reliable diagnostic capabilities. In many cases, corrosion and other forms of environmental degradation in construction materials determine the safe operating life of these vessels. If the precise operating environment of the tank materials are known, the tank can often be designed so that corrosion related failures can be minimized. However, often the operating environment of these storage tanks are not known. The precise solutions inside may change with time and residual solutions from one period of operation may contaminate the solutions added at a subsequent time and result in particularly aggressive environmental conditions for the tank materials. An accurate in-service monitoring system would provide a warning that the corrosion environment of the material has changed. It is important that this warning be recognized sufficiently early that proper corrective or remedial actions can be initiated. The same problems apply to underground pipes.
Additionally, a number of chemical processing, waste remediation and powder manufacturing operations involve the need to carefully assess the mass, density, or percent solids of a liquid slurry in underground pipes or at one or a number of depths in storage tanks, for example, outlets at different depths in the tank, as an important step in production control. Typically, this information is determined from measurements made with samples extracted from the slurry. While this technique can be successful, it is time consuming, after the fact, and not very accurate even when many samples are taken. Further complications arise when it becomes necessary to establish slurry conditions in a tank where the contents are subjected to continuous agitation. Clearly, an on-line, real time, accurate monitoring system is required for enhanced process control.
Unfortunately, it is common practice to bury or encapsulate storage or holding tanks which contain the more toxic or hazardous materials as a liquid or a slurry. Consequently, tank inspection must be accomplished from the inside. This situation is rarely satisfactory since access is limited and the monitoring system often must be protected from the tank environment. A method of detecting cracks and corrosion, as well as assessing particulate properties in a liquid medium in buried storage tanks and piping would be of tremendous value.
In U.S. Pat. No. 3,550,075 (Hilchie et al.), an acoustic transducer was used to detect fractures in casing walls in liquid containing boreholes. In U.S. Pat. No. 4,909,091 (Ellmann et al.), relating to pipe wall faults, a method for the detection of corrosion and pitting in long lengths of pipeline was described. There, a number of ultrasonic sensors were mounted within the circumference of a scraper element of an interior pipeline. The cleaning apparatus directly contacted the pipe being analyzed for corrosion.
A combination pulsing magnetic reluctance coil and ultrasonic transducer, mounted on an instrument that slides along a pipeline wall, or storage tank bottom, has also been used to measure wall thickness and determine the presence of deterioration, as taught in U.S. Pat. No. 4,418,574 (Flournoy). Eddy current probe systems generating a plurality of frequencies to detect flaws at different depths in metallic conduits were taught in U.S. Pat. No. 4,855,677 (Clark, Jr. et al.), and a variety of ultrasonic probe carriers for nondestructive inspection of long lengths of tubes, are also known and taught in U.S. Pat. No. 4,189,944 (Day) and U.S. Pat. No. 4,388,831 (Sherman).
U.S. Pat. Nos. 4,856,337 and 4,955,235 (both Metala et al.) taught a probe carrier system for combined ultrasonic and eddy current inspection of small tubes, primarily metal heat exchanger tubes of steam generators. In these two inventions, the apparatus included a housing which was insertable within the tube to be inspected, and a rotatably mounted probe carrier, where the probes were ultrasonic emitters, and where a pancake eddy current probe was also included for inspection by means of an electromagnetic field. A system for driving such an inspection probe helically within a steam generator tube was taught in U.S. Pat. No. 4,992,735 (Cullen et al.) and U.S. Pat. No. 5,025,215 (Pirl).
In the area of particle concentration measurement, U.S. Pat. No. 4,706,509 (Riebel), taught an apparatus for measuring the concentration of solid particles and particle size distribution in a suspension, by generating ultrasonic waves of several frequencies, where absorption of the waves by various sized particles was measured for each frequency. Similarly, in the area of ore slurries, U.S. Pat. No. 3,779,070 (Cushman et al.), taught transducer emission of two beams of ultrasonic energy, each having different frequencies, through a continuously flowing slurry. Larger particles caused a greater loss or attenuation of the transmitted signals. Changes in attenuation were determined by sensing the amplitude of the ultrasonic signal(s) that had passed through the suspension of particles and comparing it with the known amplitude of the transmitted signal(s) in water.
While these various apparatus solve their individual problems, none provide a simple and economical means to measure slurry or suspension properties while at the same time providing capability to monitor the structural integrity of buried holding tanks containing such slurries, suspensions, or other liquid fluids. What is needed is a dual monitor for checking storage tank damage and slurry or suspension properties, capable of operation in one or the other or both modes. It is one of the main objects of this invention to provide such a combined storage tank damage and particulate suspension properties monitor.